In the recent trend toward more convenient daily life and more diverse lifestyles, the demand and preference for cans and plastic containers for food and drink are increasing, as is implied by the compact portable PET bottle particularly favored by consumers. Besides, from the viewpoint of improving the safety supervision of food and drink, it is important to perform a retort processing (a high-temperature heating processing) for heat sterilization after packing and sealing a food or drink in the can or plastic container.
The production step of performing a retort processing with a retort apparatus after packing and sealing a food or drink is essential for killing putrefying bacteria and preventing the food or drink from becoming rotten during the shelf life by heating the food or drink to high temperature and for allowing long storage of the food or drink without quality or flavor change by sterilizing bacteria. Therefore, by applying a label that proves that the retort processing is completed to the product collecting box or using heat sensitive paper that changes color as a result of heating, for example, the production operator has to confirm and closely supervise that the product has been subjected to the retort processing in the production process.
However, such confirmation and supervision are a significant burden on the production operator, and perfect confirmation and supervision are not always ensured, so that a product that has not been subjected to the retort processing can be accidentally shipped. In such a case, the food or drink can change the flavor, and if the food or drink becomes rotten, it is a serious problem for the producer and the consumer.
In order to prevent such a problem while removing the burden on the production operator involved in confirmation and supervision of whether the retort processing of the product is completed and increasing the production efficiency, the retort processing step can essentially be automated and incorporated in the production line. However, the retort processing typically requires a temperature of 100 degrees or higher and takes several minutes or longer, and the conditions are delicate and hard to control. In addition, the conditions for the retort processing vary with the kind of food or drink, so that it is not easy to reliably and properly perform the retort processing in the automated retort processing step. In addition, if a trouble occurs in the automated retort processing step or other steps in the automated production line, a product that has been insufficiently subjected to or has not been subjected to the retort processing can be accidentally produced and shipped after trouble shooting by the production operator and restart of the production line.
In view of such circumstances, there have been developed various kinds of techniques for preventing the problem described above, removing the burden on the production operator involved in confirmation and supervision of whether the retort processing of the product is completed and strictly and reliably confirming and supervising completion of the retort processing of the product.
According to a basic and relatively simple technique, a heat-sensitive ink material that changes or loses color at the high temperature of the retort processing is used. Specifically, for example, there have been proposed an inexpensive retort packaging sheet that comprises a laminate packaging sheet and an indicator ink layer capable of changing color by steam at a sterilization temperature formed between films of the laminate packaging sheet (see the patent literature 1) and a defective can detecting method that involves printing production data (internal pressure of the can or the like) on the can using a fluorescent heat-sensitive ink capable of changing or losing color under a predetermined heat sterilization condition, confirming completion of the retort processing based on comparison of the color of the heat-sensitive ink between before and after sterilization, and reading the production data by irradiating the can with excitation light (see the patent literature 2).
Furthermore, as a technique of confirming completion of the retort processing by detecting a temperature change by using a bimetal, there have been proposed a technique of confirming completion of the retort processing by obtaining the temperature history by using a temperature history sensor that incorporates a heat-sensitive member that is displaced by two kinds of bimetals that move in response to a temperature change (see the patent literature 3) and a sterilization checking apparatus that automatically detects a positional change of a thermo checker that is activated by movement of a bimetal in response to a temperature change and checks and confirms the status of sterilization of the container in which drink is sealed (see the patent literature 4).
Furthermore, as a technique that involves installation of a heat processing completion confirming system, there has been proposed a heat processing completion confirming system for a heat processing process that involves installing a sensor capable of retaining the processing temperature history on a holder near the retort processing apparatus in the production line through which the product to be subjected to the retort processing is carried and checking the heating history with a storage and display unit covering the sensor (see the patent literature 5).
Of the various types of techniques, the technique that uses a heat-sensitive ink material capable of changing or losing color at the high temperature of the retort processing requires a special and relatively expensive heat-sensitive ink, a printing apparatus and a printing step, and the printed ink can peel. Furthermore, although it is possible to confirm that the temperature of the retort processing is reached, it is not always possible to confirm whether the retort processing is adequately performed.
The technique of confirming completion of the retort processing by detecting a temperature change by using a bimetal requires a temperature history sensor incorporating a special heat-sensitive member or a sterilization checking apparatus that detects a positional change of a thermo checker activated by movement of the bimetal, and thus, the cost of equipment for the production process is significantly high.
Furthermore, the technique that involves installation of a heat processing checking system for a heat processing process requires installation of a particularly expensive special heat processing completion confirming system provided with a sensor capable of retaining the processing temperature history and a storage and display unit, although it is possible to confirm that the retort processing is adequately performed.
Any of the conventional techniques described above are a method or apparatus for confirming completion of the retort processing developed from the viewpoint of the producer. Only in the patent literature 2, a slight mention is made of confirmation of completion of the retort processing by the consumer (Abstract and Paragraph 0007). However, the confirmation of completion of the retort processing based on comparison of the color of the fluorescent heat-sensitive ink is not always an easy and simple confirmation method for the consumer, and the consumer cannot confirm whether the retort processing is adequately performed.
Recently, as a container for which the producer and the consumer can easily confirm completion of the retort processing of the product, there has been proposed a bottle for food made of polyethylene terephthalate resin formed by biaxial stretch blow molding, the bottle having a part that is not stretched at the center part of the bottom part thereof that is whitened by crystallization caused by a heating processing, such as the retort processing, after filling with the content. According to this production process supervising method, whitening of the center part of the bottom part of the bottle due to crystallization caused by the heating processing is used as an indicator for confirming completion of the heating processing of the product (see the patent literature 6). However, the bottle for food made of polyethylene terephthalate resin probably cannot resist the high temperature of the retort processing for a long time, so that this technique is not always easy to put to practical use.    Patent literature 1: Japanese Patent Laid-Open No. 2003-334897 (Abstract)    Patent literature 2: Japanese Patent Laid-Open No. 2004-203464 (Abstract and Paragraph 0007)    Patent literature 3: Japanese Patent Laid-Open No. 2003-344174 (Abstract)    Patent literature 4: Japanese Patent Laid-Open No. 2002-104336 (Abstract and Paragraphs 0016 and 0020)    Patent literature 5: Japanese Patent Laid-Open No. 2002-142736 (Abstract)    Patent literature 6: Japanese Patent Laid-Open No. 2005-313932 (Abstract and Claims)